1. Field of the Invention
The present invention relates to a process for production of a bag having linear ribs which uses a resin film having linear ribs and enhanced stiffness as the material thereof and is suitable for production of a self-supporting bag.
2. Description of the Related Art
Bags made of resin films to which the self-supporting property is provided by the structure of a gazette folding at the bottom have heretofore been known.
However, the upper part of these bags has a lower stiffness even though the bottom part has the self-supporting property provided by the structure of gazette folding. Particularly when the bag contains a small amount of contents, the bag shows inferior stability of the self-supporting property, such as bending down of the upper part of the bag.
To solve the problem described above, the present inventors already proposed a self-supporting bag having higher stability in shape, in which a multi-layer resin film containing a resin layer of a high melting point and higher toughness is used, a shape having a constriction at the upper part of the bag is adopted and ribs are formed at the surface of the film for the bag to enhance the self-supporting property (International Patent Application Laid-Open No. WO 92/21581).
The bag made of a multi-layer resin film and having ribs proposed above uses a material sheet of a multi-layer resin film containing a layer of a heat adhesive resin having a low melting point and heat sealing property at the inside of the film and a layer of a resin having a high melting point and higher toughness at the outside of the film.
In this technology, it is difficult to mold the layer of a resin having a high melting point in the material sheet of the multi-layer resin film into the form of ribs. Thus, ribs are formed on the inner face of the film for a bag by varying the thickness of the layer of a heat adhesive resin by pushing a heated mold against the layer of a heat sealing resin having a low melting point while the layer of the resin having a high melting point is kept in a fiat shape. Because of this situation, the ribs herein are formed on the inner side of the bag in a corrugated shape.
The ribs formed on the inside of a bag in a corrugated shape have a shortcoming that the effect of enhancing stability of the shape of a bag is insufficient because the shape of the bag is convex to the outside and a ridge of a rib protruded to the inside has small resistance to an inward deformation of the bag.
As described above, when a film forming a bag has good stiffness, handling of the bag, such as discharge of contents and storage of the bag, can be conveniently made not only when the bag is a self-supporting bag but also when the bag is a simple packaging bag. Thus, a bag having linear ribs which is formed from a film having ribs and still more improved stiffness is generally useful.